Thursday, March 24, 2016

A first look at the data we collected

It's been a week since our cruise came to end. Despite all the fun we had onboard, after 38 days at sea, most people were aching to be back on dry land. Within minutes of clearing port customs, the science party and crew started scattering in all directions - some to beach, some to the airport, most to the pub.

The Revelle back in port.

After many hugs and handshakes, we all said our goodbyes and went our separate ways. Most of us are now back in our respective hometowns (I am writing from Seattle), but a few restless souls returned to the ship for the second leg of our cruise. They will pick up where we left off and continue northwards into the Bay of Bengal. Their final stop be in Phuket, Thailand. I can't imagine what it is like to do these types of cruises in succession. It's been almost two weeks since I collected my last sample, but I feel like my body is still recovering from the grueling experience. I am sure this line of work gets easier with experience, but I still have to admire the people who do these research cruises for a living.

As the saying goes, all good things must come to end. Now that the cruise is over, there is really not much left to add to this blog. To wrap things up, I plan to post all the content that I couldn't share from ship. These are mostly videos of us doing deck work. But before I do that, I would like to showcase some of the data we collected on our cruise. The whole point of our cruise was to collect data, so I would be negligent to not mention them in this blog.

Figure 1: A schematic of our cruise track and station locations. Red arrows show our transit routes, where we did not stop to collect data. We ended up doing 83 of the 89 stations we originally hoped to do. Pretty darn amazing considering all the setbacks we faced.

Just to recap, on our cruise, we completed 83 stations out of the 89 we originally set out to do. This is a remarkable accomplishment considering the enormous obstacles we had to overcome - not the least of which is losing our primary rosette. All the data we analyzed onboard the ship will become publicly available once they pass through quality control. These data will eventually be hosted on CCHDO, an online database that hosts CTD and bottle data for all the CLIVAR/GO-SHIP hydrographic cruises.

We also successfully deployed six SOCCOM floats on the cruise. All of our floats have self-activated and have completed several profiles of the upper ocean. These data are already publicly available via the SOCCOM website.

To help contextualize the float data, I will first present some of the hydrographic data we collected on our cruise. These are data we obtained from our rosette deployments. Below, I show depth-latitude1 plots of temperature2, salinity, oxygen and nitrate for stations 1-82 on our transect. The faint gray dots show where we collected water samples at each station. The red dashes highlight the stations where we deployed Argo floats.

Figure 2: Plots showing depth-latitude sections of ship-based measurements of temperature, salinity, oxygen and nitrate from the 2016 I8S cruise. Station numbers are shown at the top of each plot; float deployment locations are highlighted in red. These plots show data for the full depth of the ocean. The only exception is at station 37, where the cast prematurely ended at 2000m due to mechanical issues with the winch. These data are preliminary and have yet to undergo complete post-processing.

Figure 3: Same as the previous figure but with a zoomed in look at the upper 500 meters(m).

The plots above show how temperature, salinity, oxygen and nitrate vary from north to south in the Southern and southern Indian ocean. Starting with the most obvious, we see that upper ocean temperatures decrease from north to south. This is most clearly shown in the plots of the upper 500m (Figure 3). The salinity data show a similar pattern. Combined, these relationships tell us that the Southern Ocean is generally colder and wetter than the southern Indian Ocean. These are well known facts. Let's dig a bit deeper.

This transition between the cold, wet Southern Ocean and the warm and salty southern Indian occurs over a relatively short distance. For example, between 50oS and 43oS, ocean temperatures in the upper 500m differ by about 10 oC. In the open ocean, this constitutes a huge jump in a temperature. We call these jumps fronts. Like their atmospheric counterparts, ocean fronts are associated with enhanced mixing, turbulence and eddy activity. If you take a closer look at the transition region between roughly 50oS and 43oS, you will notice alternating bands in the different water properties. These are the signatures of strong eddies, which are large vortices (think ocean tornadoes). These eddies are good at transporting water masses over large distances and they generate a lot of mixing in their vicinities. Ocean eddies express themselves as bumps or depressions in the sea surface. We can observe these surface expressions via satellite altimetry, an example of which is shown below.

Figure 4: Plot of sea surface height anomalies on February 24, 2016. This is a snapshot of the eddy-field through which we transited. The numbered labels represent our station locations. Plot credit: Viviane Mendez.

Oxygen and nitrate concentrations show a trend that is opposite to that of temperature and salinity. That is, oxygen and nitrate concentrations in the Southern Ocean are higher than those than in southern Indian Ocean. The high nitrate concentrations in the Southern Ocean has to do with the large scale upwelling of deep water around Antarctica. This something I described in more detail ins a previous post. Oxygen concentration is higher in the Southern Ocean largely because cold water can absorb more oxygen gas than warm water.

Figure 5: A blown-up perspective of the oxygen data shown in Figure 2. I have added annotations to highlight the upwelling of deep water in the Southern Ocean, and the formation and spreading of Antarctic Bottom Water (AABW). This plot shows how we can use oxygen data to infer circulation pathways in the deep ocean.

Even though oxygen and nitrate do not directly influence ocean dynamics, they tell us a lot about how the ocean moves. We can use these variables as tracers to visualize the ocean's internal circulation. Let's take oxygen for example. Above, I show a zoomed-in plot of the oxygen data from Figure 2. I have also added annotations to highlight the major overturning circulation pattern of the Southern Ocean.

The spatial patterns of oxygen concentration reveal important features of the overturning circulation of the Southern Ocean. For example, we can trace the path of low oxygen water from the intermediate depths southern Indian Ocean as it moves southward and rises to the near surface layers of the Southern Ocean. Additionally, this oxygen data reveal the formation and spreading of Antarctic Bottom Water (AABW). AABW is created during sea-ice formation in certain locations around Antarctica. As sea ice forms, it leaves behind a residue of highly saline water called brine. This brine is cold, salty and much denser than all the seawater around it. As a result, it sinks to the bottom ocean, carrying with it high concentrations of oxygen from the surface. This is why the deepest waters of the global ocean have higher oxygen levels than the waters at intermediate depths.

There is a lot more to be said about this dataset (people have spent their entire careers analyzing these data), however I will now switch focus to our float data. We deployed our floats at stations 11, 25, 36, 41, 48 and 56. These locations represent different dynamical and biological regimes of the Southern Ocean. The nice thing about deploying floats at a CTD/rosette station is that it allows to compare the initial float data with shipboard measurements from the rosette. Shipboard data are of the highest quality available, so they provide the best benchmarks for our floats.

Figure 6: Comparison of the first profile from float 9602 (aka Eep) with shipboard data from the station where the float was deployed. The solid lines represent the float data. The other lines represent the CTD/bottle data from station 36. The shipboard nitrate and oxygen measurements were obtained from discrete bottle samples, collected at 36 different depths in the ocean. These are represented by circles.

The first comparison is for float 9602, otherwise known as Eep. Eep was deployed at station 36, which (at the time) was in the center of a very active eddy field (see Figure 4). Several hours after Eep made its splash, it reported its first profile of the upper 2000m of the ocean. The plot above compares Eep's inaugural profile with the shipboard data we collected at station 36. Overall, the two data sources compare remarkably well. The floats were able to capture the general profile patterns of the temperature, salinity, oxygen and nitrate at this particular location. There are notable disagreements, but the magnitude of those discrepancies are only a few percent of the measured values. I should add that both the float and ship-based data are still raw and will undergo further processing.

Figure 7: Like Fig. 6 but for float 9637 aka Z-Pod. These comparisons, while not perfect, are very encouraging and show that our floats are working well.

The second comparison is for float 9637, aka Z-Pod. We deployed Z-Pod at station 41, which was also situated in the middle of a very active eddy field. Like Eep, Z-Pod did a pretty good job of capturing the basic profile patterns of temperature, salinity, oxygen and nitrate at this location. As before, the comparisons are not perfect, but this could be due to natural processes. For example, being in active eddy field, the float could have easily drifted into an eddy, carrying water from a different region. However, some of these discrepancies are bound to be due to instrument error and bias. But at this stage, all we can do is speculate. More analysis needs to be done

I have done similar comparisons for the other floats and the stories are the same. The floats are working well and they are reporting reasonable data, which is very encouraging.

Again, there is a lot more to be said about this data but I will end my discussion here. I am really excited of all this new data and I look forward to monitoring these floats over the next few years. Even though the cruise is over, my work is just beginning.

-EW

1 On the y-axes, I actually plotted pressure in units of deci-bars (db) instead of depth in meters (m). This is a common practice in physical oceanography, for reasons I won't get into. It just so happens that ocean pressure (in deci-bars) and depth (in meters) are numerically similar. For example, in the Southern Ocean, 1000 db is roughly equal to 990 m. Therefore, for the purposes of this discussion, you can think of depth (m) and pressure (db) as being interchangeable. ↩

2 Another technical side note: Here, I actually plottedpotential temperature which is slightly different from normal temperature. This again has to do with pressure. Potential temperature accounts for the fact that water warms under compression. This effect creates the false impression the ocean is being warmed from below. By using potential temperature, we remove the effect of warming due to compression.↩